1. Field of the Invention
This invention relates to a process for the production of a low metals content coke by integration of fluid coking and delayed coking.
2. Description of the Prior Art
Fluid coking is a well known process which may be carried out with or without recycle of the heavier portion of the fluid coking zone effluent. As is well known in the art, the fluid coking process, shown, for example in U.S. Pat. No. 2,881,130, which is hereby incorporated by reference, uses a fluid coking vessel and an external heating vessel. A fluid bed of solids, preferably coke particles produced by the process, having a size in the range from about 40 to about 1000 microns is maintained in the coking zone by the upward passage of a fluidizing gas, usually steam, injected at a superficial velocity usually between 0.3 and 5 feet per second. The temperature in the fluid coking bed is maintained in the range of from about 850.degree. F. to about 1200.degree. F., preferably between 900.degree. F. and 1100.degree. F. by circulating solids (coke) to the heating vessel and back. The heavy oil to be converted is injected into the fluid bed and upon contact with the hot solids undergoes pyrolysis evolving lighter hydrocarbon products in vapor phase, including normally liquid hydrocarbons, and depositing a carbonaceous residue (coke) on the solids. The turbulence of the fluid bed normally results in substantially isothermal reaction conditions and thorough and rapid distribution of the heavy injected oil. Product vapors, after removal of entrained solids, are withdrawn overhead from the coking zone and sent to a scrubber and a fractionator for cooling and separation.
Delayed coking is a well known process in which a hydrocarbonaceous oil heated to coking temperature is passed into a coking drum to produce a vapor phase product, including normally liquid hydrocarbons, and coke. The drum is decoked, for example, by using high pressure water jets. See Hydrocarbon Processing, Sept. 1978, page 103.
It is known to prepare a low ash content coke from a high ash content hydrocarbonaceous liquid feed by subjecting the liquid feed to a primary thermal treatment under mild coking conditions at temperatures ranging from 950.degree. to 1200.degree. F. and passing a high boiling portion of the effluent to a second thermal treatment zone under more severe conditions at 950.degree. to 1600.degree. F.
It is also known to form a high grade needle coke by heating petroleum residuum to about 350.degree. to 550.degree. C. to remove components which readily form an insoluble phase and coking the remaining residuum.
It is known to produce a high grade petroleum coke by subjecting the oil to a first delayed coking under relatively mild conditions and thereafter subjecting the uncoked heavy residual oil to a second delayed coking under relatively more severe conditions. See U.S. Pat. No. 3,959,115.
A two-stage delayed coking process is known for producing an inorganic contaminant-free coke. See U.S. Pat. No. 3,769,200.
Two-stage fluid coking processes are also known. See U.S. Pat. Nos. 2,854,397; 2,879,221 and 3,671,424.
It has now been found that fluid coking followed by delayed coking will provide advantages in the production of a low metals coke product.